partial response to #163 to add retries for dls

[dpastoor]

This does 2 things:

1. it potentially allows the dl concurrency to be bound for servers that don't want to accept too many concurrent connections by setting PKGR_DL_CONCURRENCY to some integer value (or the flip side make it larger)
2. implements retries and will retry 3 times

still TODO: bubble up the failing errors to cause pkgr to completely error out sooner

@bschulth let me know if you'd like to give this a try asap and I can cut a pre-release, else will consider how to test these a little bit